Generally, a radio communication system has mobile terminals 10, radio base stations 30, and mobile communication core network 50 as shown in FIG. 1.
Mobile terminal 10 transmits and receives data such as communication traffic and control traffic to and from radio base station 30.
Radio base station 30 transmits and receives data to and from mobile terminals 10 and mobile communication core network 50 connected through wired links 40. Around radio base station 30, radio cell 35 is arranged as an effective range in which mobile terminals 10 each have radio link 20 with radio base station 30. Radio base station 30 manages radio cell 35 arranged in its own periphery, whereas mobile terminal 10 establishes a connection with radio cell 35 that radio base station 30 manages and transmits and receives data to and from radio base station 30.
Mobile communication core network 50 is composed of an exchanger and a server machine (not shown) and is connected to radio base stations 30 through respective wired links 40 and is connected to external network 70 through wired link 60 so as to transmit and receive data to and from radio base stations 30 and external network 70.
In such a radio communication system, radio parameters of radio base station 30 need to be appropriately set up so as to satisfy predetermined communication qualities in all the service areas in which services are provided. Examples of such radio parameters are total transmission powers of radio base stations 30, tilt angles on the vertical and horizontal planes of a radio base station antenna, a neighbor cell list, and a handover threshold.
In the following description, it is assumed that as radio parameter, a neighbor cell list is provided in radio base station 30.
Here, the neighbor cell list will be described.
In the radio communication system, radio cells are arranged on the plane of all the service area so as to provide services.
When mobile terminal 10 moves from the connected radio cell to another radio cell, mobile terminal 10 performs a so-called handover, which is a process that switches the connected radio cell to another radio cell. In this process, mobile terminal 10 is pre-instructed to measure radio communication qualities of the connected radio cell and its neighbor cells and to report measured radio communication qualities of the connected radio cell and the neighbor radio cells to radio base station 30 that manages the connected radio cell if the radio communication quality of the connected radio cell deteriorates. When the radio communication quality is reported from mobile terminal 10 to radio base station 30, radio base station 30 determines a handover destination radio cell.
To reduce the load imposed on mobile terminal 10 and process the handover quickly, a method that designates specific cells of neighbor cells as candidates of a handover destination radio cell is generally used. The neighbor cell list shows candidates of a handover destination radio cell. The neighbor cell list is generated when a communication carrier registers radio cells to each radio cell, and the neighbor cell list is reported to mobile terminals 10 by radio base station 30 through a downlink channel.
In the following description, radio cells that have been registered to a neighbor cell list are referred to as listed cells, whereas radio cells that have not been registered thereto are referred to as detected cells.
When the radio communication quality is reported from mobile terminal 10 to radio base station 30, radio base station 30 determines a handover destination radio cell from the listed cells whose radio communication qualities have been measured by mobile terminal 10. Therefore, mobile terminal 10 cannot perform a handover to detected cells whose radio communication qualities have not been measured by mobile terminal 10 and who radio communication qualities have not been reported to radio base station 30. Consequently, if radio cells have been omitted from the neighbor cell list, since mobile terminal 10 cannot perform a handover to an appropriate radio cell, a call may be abnormally terminated or a handover to an inappropriate radio cell may result in deterioration of radio communication qualities. Thus, it is important to generate a neighbor cell list in which radio cells have not been omitted so as to ensure good radio communication qualities.
On the other hand, an upper limit (LMax) of the number of radio cells that can be registered to a neighbor cell list has been set up so as to reduce the load imposed on a channel through which the neighbor cell list is reported and in order to reduce the load imposed on mobile terminal 10 that needs to measure radio communication qualities of radio cells and report them to radio base station 30. Thus, it is necessary to preferentially register radio cells, that are likely to contribute to an improvement of communication qualities, to the neighbor cell list so as to register major radio cells to the neighbor cell list.
Generally, setting a neighbor cell list requires an advanced technique. The neighbor cell list has been optimized based on field test results in the service area of the radio communication system.
However, in recent years, a neighbor cell list optimizing system that autonomously optimizes a neighbor cell list during the operation of the radio communication system has been under consideration.
FIG. 2 is a schematic diagram showing an example of a structure of a radio communication system that is provided with a neighbor cell list optimizing system.
The radio communication system shown in FIG. 2 has mobile terminals 10, radio base stations 30, mobile communication core network 50, and neighbor cell list optimizing system 90. In FIG. 2, similar components to those in FIG. 1 are denoted by similar reference numerals and their description will be omitted in the following.
Neighbor cell list optimizing system 90 is connected to radio base stations 30 through wired links 80 and to mobile communication core network 50 through wired link 85. Neighbor cell list optimizing system 90 optimizes neighbor cell lists on which radio base stations 30 manage radio cells 35.
Non-patent Literature 1 describes a neighbor cell list updating method that the neighbor cell list optimizing system performs.
FIG. 3 is a schematic diagram illustrating the neighbor cell list updating method described in Non-patent Literature 1.
In Non-patent Literature 1, it is assumed that a mobile terminal measures the received power of a pilot signal transmitted from a radio base station, as radio communication quality, and reports the measured received power to the radio base station.
In the neighbor cell list updating method described in Non-patent Literature 1, the number of handover attempts (a) to each listed cell registered in the neighbor cell list is tabulated. In addition, the number of reports (n) received from mobile terminals stating that the received power of a pilot signal is equal to or higher than a threshold regarding each detected cell is tabulated.
The neighbor cell list is updated in such a manner that detected cells in which the number of reports (n) is equal to or higher than a threshold (ThAdd) are registered in the neighbor cell list in descending order of the number of reports (n) and that listed cells in which the number of attempts (a) is equal to or lower than a threshold (ThDel) is deleted from the neighbor cell list.